The present invention relates to gas flow measuring apparatus and more particularly to an apparatus for measuring, for example, the amount of air flow to an engine.
In the past, there has been proposed an apparatus in which a flow measuring tube is disposed in the intake pipe of an automobile engine, and an electric heater and temperature dependent resistors, which each consist of a platinum resistance wire, are disposed in the flow measuring tube, whereby the flow rate of air (the gas to be measured) is measured in accordance with the output signals of the heater and the resistor.
While this apparatus has the advantage of ensuring accurate flow measurement with a compact and simple construction, there is a disadvantage in that since a very fine platinum resistance wire is used for the temperature dependent resistors and the resistors are used by suspending them in the air stream, there is a problem from the strength point of view when the platinum resistance wires are subjected to a shock due to a backfire (if the ignition timing or the valve timing of the intake and exhaust valves are adjusted improperly in the engine combustion mechanism, a phenomenon, in which the fuel and air are burned explosively in the intake pipe by the flame in the combustion chamber, occurs and this phenomenon is called "backfire").
Further, in the measuring principle the control is initiated only when the heat of the electric heater is applied to the temperature dependent resistor, and consequently from the control point of view it is essential that the heat of the electric heater is rapidly transmitted to the temperature dependent resistor. However, the known apparatus is disadvantageous in that since the electric heater, the first temperature dependent resistor and the second temperature resistor are each mounted on a separate support, there is a limit from the distance point of view for arranging the electric heater and the first temperature dependent resistor, which receives the heat from the former, in close proximity to each other.
Another disadvantage is that since the electric heater and the first temperature dependent resistor adapted to receive the heat from the former are each disposed on a separate support, the heat of the electric heater is not entirely applied to the first temperature dependent resistor and part of the heat is taken by the support, thus causing a flow measurement error corresponding to the heat loss and thereby deteriorating the measurement accuracy.